Heretofore, exhaust gas discharged from combustion chambers of cylinders of a multi-cylinder internal combustion engine have been collected by a collecting pipeline, which is configured by bundling pipes of a number corresponding to the number of cylinders. Additionally, in such a multi-cylinder internal combustion engine, an exhaust gas sensor for detecting the state of exhaust gas such as a temperature sensor and an air-fuel ratio sensor is provided in a part of the collecting pipeline where streams of exhaust gas from the cylinders merge. However, in this case, in order for a single exhaust gas sensor to uniformly detect the state of exhaust gas discharged from the cylinders, the exhaust gas sensor needs to be provided is a position where the exhaust gas from, the cylinders evenly hit a detector of the exhaust gas sensor.
For example, in order to allow the exhaust gas from the cylinders to evenly hit a detector of an exhaust gas sensor, Japanese Utility Model Registration Application Publication No. Sho 62-126512 discloses a technique in which an exhaust gas sensor is provided substantially parallel to the flow direction of exhaust gas, so that a detector of the exhaust gas sensor protrudes into the center of an exhaust gas-merging part. Meanwhile, Japanese Utility Model Registration Application Publication No. Sho 58-162225 discloses a technique in which an extension chamber is provided at the center of an exhaust manifold, the exhaust gas is introduced into the extension chamber from the right and left thereof through curved passages, and an exhaust gas sensor is provided inside the extension chamber.